Solid-state imagers, in general, comprise a plurality of photodetectors arranged in an array, such as an array of rows and columns, and shift registers, such as CCD shift registers, between the columns of the photodetectors. The photodetectors in each column are coupled, such as by a transfer gate, to their adjacent shift register so that the charge carriers generated and accumulated in the photodetectors can be selectively transferred to the shift register. The shift registers transfer the charge carriers to the read-out circuit of the imager.
One problem which has arisen in this type of solid-state imager is referred to as "blooming." If, during the accumulation period of the imager, the period when the photodetectors are receiving photons from the image and converting the photons to charge carriers, a photodetector accumulates an excess amount of the charge carriers, some of the charge carriers will overflow from the photodetector into the adjacent shift register and/or photodetector. This adversely affects the charge carriers in the shift register being transferred to the read-out circuit causing "blooming". A conventional technique used for preventing blooming, i.e. an anti-blooming technique, uses an overflow drain adjacent the photodetector with the drain being isolated from the photodetector by a potential barrier. The potential barrier between the photodetector and anti-blooming drain is lower than the barrier provided by the transfer gate between the photodetector and the CCD shift register during the accumulation period. Thus, if the charge level in the photodetector reaches a sufficient amount to raise the photodetector potential to a level above that of the barrier between the photodetector and the anti-blooming drain, additional signal carriers will be swept over into the anti-blooming drain where they are removed by the drain supply. This prevents excess charge from flowing into the shift register during the integration period and thereby prevents blooming.
Another problem which has arisen in certain types of solid-state imagers relates to controlling the exposure time of the photodetectors. For certain applications, such as still photography, it is desirable to be able to control the time that the photodetectors generate charge carriers as a result of being exposed to the image being taken. Various shuttering techniques have been developed for this purpose. However, the structure which has been required to achieve the shuttering for exposure control as well as the gating and drain required for anti-blooming control has taken up space on the imager and therefore has reduced the fill factor of the imager. Therefore, it would be desirable to have a solid-state imager which includes both a shutter for exposure control and anti-blooming features in which the number of elements required for these features is minimized to improve the fill factor of the imager and to reduce the number of electrodes required to perform these functions and thereby reduces system complexity.